The most familiar type of magentic single wall domains are referred to as magnetic bubbles. Magnetic bubbles are cylindrical-shaped areas of magnetization typically maintained in a plane of movement at a nominal operating diameter determined by a bias field antiparallel to the direction of magnetization of the bubble. The most common means for defining the plane of movement for magnetic bubbles is an epitaxial film of garnet grown on a nonmagnetic garnet single crystal as is well established in the art, although of late it has been found possible to employ bubbles in amorphous films. The film is characterized by a uniaxial anisotropy normal to the film and thus the magnetization of a bubble and the direction of the bias field align with an axis normal to the film.
U.S. Pat. No. 3,534,347 of A. H. Bobeck, issued Oct. 13, 1970, discloses a technique for moving magnetic bubbles by means of a pattern of magnetically soft (high permeability) elements, typically permalloy, on the surface of the epitaxial film in which the bubbles move. The elements are formed, by photolithographic techniques, in a repetitive pattern along an axis of bubble movement. A magnetic drive field rotating through consecutive orientations in the plane of bubble movement temporarily magnetizes those elements which have long dimensions aligned with the field. As the field rotates, different elements of the pattern become magnetized due to differing orientations of their long dimensions. By arranging the elements so that the long dimensions of consecutive elements in each period of the pattern are aligned with successive orientations of the drive field, a magnetic pole pattern can be made to move along the axis. Since a repetitive pattern is formed by the elements, the moving pole pattern is also repetitive and thus can be made to move a bubble pattern along that axis. If a binary one or a binary zero is represented by the presence or absence of a bubble, respectively, in a position corresponding to each period of the pattern of elements along the axis, information can be represented as moving along the axis as the drive field rotates. Because the bubbles are moved by the drive field without electrical connection to the garnet or to the permalloy elements, this technique of bubble propagation is commonly referred to as the "field-access" mode of operation.
The most familiar pattern of elements for operation in the field-access mode presently is a T and bar-shaped pattern although chevron, Y-bar, T-X and curvilinear geometries, such as disc-shaped and petal-shaped elements for bubble propagation have been disclosed in the art also. Typically, information defined by a bubble-no-bubble pattern is associated with a distribution of "attractive" magnetic poles generated by the elements as the field reorients in a manner to make the next succeeding pole and the next preceding pole, with respect to each bit, attractive and repulsive, respectively. As reorientation of the field occurs, the poles corresponding to the position occupied by a bubble typically become maganetically neutral. In each instance during bubble propagation, the relationships between the distribution, intensity, and timing of the next preceding, the instant, and the next subsequent poles determine a stable range over which bubble operation occurs in practice. Thus, for any given material, the geometries of the propagate elements, in general, determine these relationships.
The stable range over which a bubble device operates is usually described by a margin plot of bias field intensity versus drive (rotating) field intensity. It is well established that in any given bubble material, a stable bubble exists over a range of bias fields from a high field at which spontaneous bubble collapse occurs to a low field at which bubble strip-out occurs. This range of bias fields is typically twenty-five oersteds between, for example, eighty-eight oersteds, and one hundred and thirteen oersteds. At a high bias field, the bubble diameter is smaller than it is at a low bias field. The propagate elements are of a fixed dimension determined for an intermediate bias field at which the bubble is at some nominal diameter in order to ensure operation tolerant of variations in drive (rotating) field intensity and to allow for driving a number of memories (i.e., memory chips) with a single drive field. For a given bubble device, at a given bias field intensity and drive field, the geometry of the propagate elements thus determines the margins of operation.
Another important consideration in the fabrication of a practical bubble device is "yield", defined as that percentage of manufactured memory chips which meet specified operating criteria. Defective devices arise primarily from defects in the permalloy elements as a result of processing. Certainly, a propagate pattern which is tolerant of (operative in the presence of) defects such as missing areas or extra areas of permalloy enhances yield. A propagate pattern which enables a high yield of chips operative over prescribed margins also permits a trade-off between yield and margins. Therefore, from a practical standpoint, the geometry of the propagate elements determines the yield.
An added consideration for the design of a propagate pattern is the drive field required to achieve a desired pole strength. Of course, the lower the drive field needed, the lower the power requirements for bubble operation. But the pole strength achieved with a given drive field is determined by the geometry and bulk of the propagate elements. In general and within practical limits, for a given element thickness and shape, the larger the element the less the demagnetizing field which has to be overcome by the drive field for producing the poles which, in turn, produce bubble movement. Therefore, for a bubble of given size, the larger the element the lower the requisite drive field or the greater the pole strength for a given drive field. It should be apparent, then, that the geometry of the propagate element is an important factor also in determining the power requirements of bubble devices.
Additional factors become increasingly important as bubble devices are operated at increasingly higher frequency and as increasingly smaller bubbles are employed. For example, as increasingly higher frequencies are employed, the mobility of the bubbles in magnetic material becomes important. If the bubble movement is nonuniform during a cycle of the rotating field, for example, a bubble could be moving at close to the mobility limit of the material for a portion of the cycle and well below the limit for other portions of the cycle. Under high frequency operation, a bubble may not properly arrive at its next consecutive position in time and may, as a result, be annihilated. A relatively lower frequency might be dictated to avoid such a difficulty or a relatively higher power may be necessary to drive the bubbles at a desirably higher frequency than would be required by a propagate pattern which moved the bubble at a more uniform rate. The desirability of smooth bubble movement insofar as it contributes to low drive field may be appreciated more fully when it is recognized that the rotating field advantageously is generated by a pair of field coils which are elements of a tuned circuit arrangement. Switching of tuned circuits is increasingly difficult as power requirements and/or frequency increase.
The factors relating to bubble size concern the resolution of photolithographic techniques. The period of a propagation pattern is typically four to five (nominal) bubble diameters and requires a bar and a T-shaped element, for example, to be defined in the space of each period. The present state of the art for photolithography in production is about one and one-half microns resolution with the finest detail in the propagation pattern being the spacing between say a bar and a next subsequent T-shaped element. This spacing determines the maximum resolution for the pattern and processing variations cause changes in this spacing. Consequently, a propagate element design which operates well with different spacings between elements for a given period, can be made with relatively greater capacity, or packing density, with present photolithographic techniques. It is apparent accordingly that the geometries of the propagate elements also affect the limits of the frequency of operation and the packing densities.
Detailed studies of bubble propagation indicate that the critical point during operation occurs when a bubble transfers from one propagate element to another. It is at this point in the propagate pattern that intensity, distribution and timing of the poles, and the shape, spacing and bulk of permalloy along with the mobility and uniformity of movement are most determinative of margins and yield.